I just tried and created 2000 rooms with no problems.
Could it be that you have already created those rooms and you are getting an error 403 because the room already exists ?
This may explaine the 1000 rooms limit. Because if you call listRoom() and your account has more than 1000 rooms I am only returning the first 1000 (as I said before listing room is an expensive and long operation).
So, maybe you have already created those rooms a long time ago, but you don't see them when you "list" them. That would explain why if you delete one of those rooms you can recreate it.
You can try creating rooms with random names (or guid) and you'll probably see that the operation suceeds.